Antidepressants come with a warning that suicidal thoughts can occur. Any psychiatrist or doctor that is worth anything will bring this to the patient's attention. The problem is that when suicidal thoughts occur, patients will often ignore them and will not take the steps necessary to get help.
As far as Xanax goes, again, any psychiatrist or doctor that is worth anything will factor in alcohol use before they consider prescribing the drug. The problem with that though is that patients will often lie about their alcohol use, or will continue to use alcohol even though they are told not to. People who have a history of addiction should not be prescribed this drug.
Neurontin is generally prescribed for some types of seizures. It is also sometimes prescribed as a sleep aid. Neurontin is rarely prescribed for schizophrenia, and when it is, it is in addition to an antipsychotic, and only when other medications don't work. Neurontin has little to no abuse potential.
People who respond poorly to these medications, or abuse them either have some other sort of disorder going on, or are abusing drugs and/or alcohol, which interferes with the medications that are prescribed.
As far as Chris Benoit goes, he was known to abuse steroids, which increases testosterone in the body. When he died, he was found to have 10 times the normal amount of testosterone in his body. Increased testosterone = increased aggression.